Electrical resistor



Oct. 19, 1965 A. G. OWEN ELECTRICAL RESISTOR Filed July 30, 1962 UnitedStates Patent O 3,213,401 ELECTRICAL RESISTOR Albert G. Owen,Monroeville, Pa., assignor to Mosebach Manufacturing Company,Pittsburgh, Pa., a corporation of Pennsylvania Filed July 30, 1962, Ser.No. 213,176 6 Claims. (Cl. SSS-58) This invention relates to anelectrical resistor and has to do particularly with an electricalresistor having means for dissipating heat. The invention is concernedwith a novel design and arrangement of resistor elements in relation tothe direction of a cooling draft which is imposed on the resistorelements tending to equalize the operating temperature of the resistorelements and hence greatly increase the efficiency of the electricalresistor.

Electrical resistors provided with means for dissipating heat, i.e.,means for cooling the resistor elements, have a wide variety of uses.One prominent use of such electrical resistors is in railroadlocomotives employing dynamic braking systems. Dynamic braking systemsare used on downgrade runs to relieve the loading and resultant wear andtear on the air operated shoe-type brakes. The electrical energygenerated by the dynamic braking System causes heating of the resistorelements which are subjectedA to a cooling draft which is generallyinduced by the movement of the train but may be otherwise created.

An electrical resistor of the type referred to comprises a plurality ofresistor elements arranged in spaced apart relationship in atwodimensional field, the resistor elements being electrically connectedto one another and to a source of current which may be the generator ofthe dynamic braking system, and the cooling draft is directed throughthe two-dimensional field of resistor elements. Normally the resistorelements are elongated and arranged in generally parallel relationshiplike the trees in a forest and the cooling draft moves through theresistor elements generally at right angles thereto as a wind movesthrough the trees of a forest. The cooling draft is initially relativelycool but its temperatur-e is increased as it passes through the field ofresistor elements since heat in the resistor elements is abstracted bythe cooling draft and carried away. Hence the cooling draft becomesprogressively hotter as it passes through the field of resistorelements,

The resistor elements may be in the form of resistor coils. The coils asmounted in the electrical resistor are coreless and normally theirsuccessive convolutions are spaced apart. Heretofore it has been thepractice to utilize identical coils throughout the electrical resistor.The cooling draft directed through the coils has cooled the coils firstengaged by the draft to a temperature lower than the temperature towhich the coils later engaged by the draft have been cooled, first,because of the progressively increasing temperature of the coling draftas it passes through the field of coils, and, second, because the coilsthrough which the cooling draft first passes have a shielding effectwith respect to the coils through which the cooling draft later passes,decreasing the effectiveness of the cooling draft on the lattermentioned coils. The result has been that in operation the coils moreremote from the source of the cooling draft have operated at atemperature substantially higher than the coils nearer the source of thecooling draft. A given coil can withstand only a certain temperature,after which it melts. The capacity of the electrical resistorsheretofore employed has been limited by the maximum permissivetemperature of the coils most remote from the source of the coolingdraft. Since uniform coils have heretofore been used throughout theelectrical resistor this has resulted in the coils nearer the source ofthe cooling draft, which are relaice tively cool, operating atmaterially less than theoretical potential with an overall sacrifice ofefficiency in the electrical resistor.

I have devised an electrical resistor which cures the defect of priorelectrical resistors as above explained. My electrical resistor isdesigned so that all of the resistor elements or coils of the electricalresistor operate at approximately the same temperature which enablesmaximum or optimum efficiency to he derived from each resistor elementor coil with an unprecedented increase in the overall eiiiciency of theelectrical resistor. I design the respective resistor elements 'or coilsof different characteristics so that the electrical resistance of theresistor elements nearer the source of the cooling draft is less thanthat of those more remote from the source of the cooling draft, whichresults in a tendency (apart from the cooling draft) of the resistorelements closer to the source of the cooling draft to operate at ahigher temperature than resistor elements more remote from the source ofthe cooling draft. I take into consideration the fact that thetemperature of the cooling draft progressively increases as it passesthrough the field of resistor elements or coils. I further take intoconsideration the shielding effect of the coils nearer the source of thecooling draft upon the coils more remote from the source of the coolingdraft and design the coils to obviate to a considerable extent theshielding tendency. As a result my electrical resistor operates at arelatively uniform temperature throughout, enabling for the first timerealizing of `optimum efficiency.

I provide an electrical resistor comprising a plurality of resistorelements arranged in spaced apart relationship in a two-dimensionalfield, means electrically connecting the resistor elements to oneanother and to a source of current so that the resistor elements areheated upon the passage of current therethrough and means directing acooling draft against the resistor elements, the electrical resistanceof resistor elements subjected to the cooling draft when such draft isrelatively cool being less than the electrical resistance of resistorelements subjected to the cooling draft when such draft is relativelyhot so as to tend to equalize the operating temperature of the resistorelements. It is generally preferable to arrange the resistor elements inrows and to direct the cooling draft generally transversely of the rowsof resistor elements, the electrical resistor being designed so that theelectrical resistance of resistor elements of a row relatively near thesource of the cooling draft is less than the electrical resistance ofresistor elements of a row relatively remote from the source of suchdraft.

As above indicated the resistor elements may be and in most instancespreferably are in the form of coils. The cooling draft is directedthrough the coils and I preferably provide the effective length andhence the electrical resistance of coils subjected to the cooling draftwhen such draft is relatively cool to be less than the effective lengthand hence the electrical resistance of coils subjected to the coolingdraft when such draft is relatively hot so as to tend to equalize theoperating temperature of the coils.

I find it desirable to make the axial dimension of the coilssubstantially uniform and to make the pitch of coils subjected to thecooling draft when such draft is relatively cool greater than the pitchof coils subjected to the cooling draft when such draft is relativelyhot. I may provide coils of substantially uniform effective length withthe transverse crossasectional area of the coiled element forming coilssubjected to the cooling draft when such draft is relatively coolgreater than the transverse crosssectional area of the coiled elementforming coils subjected to the cooling draft when such draft isrelatively hot.

In my electrical resistor the pitch of the respective coils and thetransverse cross-sectional area of the coiled elements forming therespective coils are preferably selected so that the electricalresistance of coils subjected to the cooling draft when such draft isrelatively cool is less than the electrical resistance of coilssubjected to the cooling draft when such draft is relatively hot; alsopreferably so that coils subjected to the cooling draft when such draftis relatively cool have relatively wide openings between theirconvolutions in contrast to coils subjected to the cooling draft whensuch draft is relatively hot to reduce obstruction to the cooling draftin its passage to coils subjected to the cooling draft when such draftis relatively hot.

The pitch of coils in a row relatively near the source of the coolingdraft may be greater than the pitch of coils in a row relatively remotefrom the source of such draft so that the electrical resistance of thecoils of the first mentioned row is less than the electrical resistanceof the coils of the second mentioned row, and also the coils of thefirst mentioned row may have relatively wide openings between theirconvolutions to reduce obstruction to the cooling draft in its passageto the coils of the second mentioned row.

Preferably the resistor elements are arranged in rows and columns, therows extending transversely of the direction of the cooling draft andthe columns extending generally in the direction of the cooling draft,the elements of the respective rows in each column being electricallyconnected together in parallel or multiple (parallel and multiple aresynonymous in the context of electric circuitry; multiple is used in theclaims to avoid confusion with parallel7 used in defining the spatialrelationship of the colums), the columns being electrically connectedtogether in series.

Other details, objects and advantages of the invention will becomeapparent as the following description of a present preferred embodimentthereof proceeds.

In the accompanying drawings I have shown a present preferred embodimentof the invention in which:

FIGURE 1 is a face view, with portions cut away, of an electricalresistor embodying my invention;

FIGURE 2 is a cross-sectional view taken on the line II-II of FIGURE 1;and

FIGURE 3 is an end View of the electrical resistor shown in FIGURES 1and 2 as viewed from the right in those figures.

The electrical resistor comprises twenty-seven coreless resistor coils 2arranged as shown in the drawings in three rows R1, R2 and R3 and ninecolumns C. The coils are mounted in a casing designated generally byreference numeral 3 having side members 4 connected together by endmembers S and the front 6 and the back 7 of the casing 3 being open toallow a cooling draft to be directed against and through the coils. Thecooling draft is directed substantially in the direction of the arrow Ain FIGURES 2 and 3 generally at right angles to the rows R1, R2 and R3of coils and in the direction of or along the columns C thereof. Thecooling draft may be created by induction or by a blower blowing thecooling air into the field of resistor coils at 6 and out at 7. In anyevent the cooling draft passes through the eld of coils in the directionindicated and abstracts heat vfrom the coils. The coils are mounted inthe casing 3 by transite spacers 8 with holes cut therein for the coilsto pass through. The coils are electrically connected together as shownin the drawings. The electrical connections at one end of the casing aredesignated `9 and the electrical connections at the other end of thecasing are designated 11D. In FIGURE 3 the electrical connections 9 areat the end of the casing nearer the eye and the electrical connectionsat the opposite end of the casing are indicated by chain lines. Thus itis seen that the coils of the respective rows R1, R2 and R3 in eachcolumn C are electrically connected together in parallel and the columnsC are electrically connected together in series. The electrical resistoris connected with the generator through terminals 11.

The cooling draft engages first the coils of the row R1, thereafter thecoils of the row R2 and finally the coils of the row R3. If all of thecoils were uniform in accordance with prior practice the coils of therow R1 would operate at relatively low temperature the coils of the rowR2 would operate at intermediate temperature and the coils of the row R3would operate at relatively high temperature for the reasons aboveexplained. To relatively equalize the temperature of operation of all ofthe coils I design the coils of the row R1 to have minimum electricalresistance, those of the row R2 to have intermediate electrical`resistance and those of the row R3 to have maximum electrica-lresistance. At the same time I prefer to reduce the shielding effect ofthe coils of the row R1 by designing those coils with maximum pitch,i.e., with maximum spacing between their convolutions, the coils of rowR2 with intermediate pitch and the coils of row R3 with minimum pitch. Ifind it desirable to use heavier wire for forming coils relatively nearthe source of the cooling draft and lighter wire Kfor forming coilsrelatively remote from the source of the cooling draft. In an electricalresistor of the type shown in the drawings I desirably form the coils ofrow R1 of No. 1 wire and the coils of rows R2 and R3 of No. 2 wire.

'Since the voltage is constant the amperage per coil in the coils of rowR1 will be greater than the amperage per coil in the coils of row R2 andthe amperage per coil in the coils of row R2 will be greater than theamperage per coil in the coils of row R3 in order that the resistanceper coil in the coils of row R1 may be minimum that in the coils in therow R2 may be intermediate and that the coils in the row R3 may bemaximum, resulting in operation of the electrical resistor atsubstantially uniform operating temperature throughout whereby maximumelectrical efficiency is attained.

The following table lists the characteristics of a typical electricalresistor designed in accordance with my invention:

In the exemplary electrical resistor the length of the coils is 29inches, all coils of the three rows being of the same length. The turnsper inch of the coils of row R1 equals 2.035. The diameter of the wireof the coils of the row R1 is .2893 inch so that the maximum possiblenumber of turns would be 29/ .2893 or approximately 100 turns (with nospace between turns). 59 equals a space factor of 1.7, the space betweencoil convolutions being about 70% of the diameter of the wire. Themaximum possi-ble number of turns of the coils of rows R2 and R3 is 29divided by .2576 (the wire diameter) or 112.5. 112.5/63 equals 1.79, thespace factor in row R2. The space factor in row R3 is 112.5/69 or 1.63.Thus in all three rows a good space Ifactor is maintained providing forfree passage of air through the coils and about the respectiveconvolutions of each coil. In the particular electrical resistorselected as an example thespace factor in row R2 is slightly greaterthan that in row R1 but it is to be borne in mind that the wire in rowR2 is smaller than that in row R1 and hence more easily cooled. Thespace factor in row R3 is less since the coils of row R3 do not shieldany other coils and hence shielding effect is not a factor in the designof the coils of row R3.

The most important consideration is the designing of the resistorelements or coils for minimum electrical resistance in row R1,intermediate electrical resistance in row R2 and maximum electricalresistance in row R3. Electrical resistance is a function of thediameter of the wire used for forming the coils and the effective lengthof such wire in the coil. Increase in wire diameter decreases electricalresistance while increase in the effective length of the wire increaseselectrical resistance so an optimum electrical resistance can be arrivedat by proper relative selection of wire diameter and effective wirelength in the coil. Also the pitch of the coil affects not onlyshielding effect but also cooling rate, a coil of greater pitch coolingmore quickly than a similar coil of lesser pitch.

Another factor which enters the picture is turbulence but that factor isnot capable of exact determination and has to be dealt withexperimentally. Also there may be a tendency in certain installationsfor the cooling draft to channel toward one side of the electricalresistor. In such cases the resistor elements at the respective sides ofthe electrical resistor may differ somewhat in wire size and pitch tothe end that the operating temperature throughout the electricalresistor is substantially the same which produces maximum electricalefficiency.

While I have shown and described a present preferred embodiment of theinvention it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied within the scopeof the Ifollowing claims.

I claim:

1. An electrical resistor comprising a plurality of resistor coilsarranged generally in rows and columns, the axial dimension of the coilsbeing substantially uniform, means electrically connecting the coils toone another in multiple and means directing a cooling draft through thecoils, the pitch of coils in a row relatively near the source of thecooling draft being greater than the pitch of coils in a row relativelyremote from the source of such draft so that the electrical resistanceof the coils of the first mentioned row is less than the electricalresistance of the coils of the second mentioned row and also the coilsof the first mentioned row have relatively wide openings between theirconvolutions to reduce obstruction to the cooling draft in its passageto the coils of the second mentioned row, so as to tend to equalize theoperating temperature of the coils when a current of electricity ispassed therethrough.

2. An electrical resistor comprising a plurality of resistor elementsarranged generally in rows and columns, the elements of the respectiverows in each column being electrically connected together in multiple,the columns being electrically connected together in series, and meansdirecting a cooling draft against the resistor elements generallytransversely of the rows of resistor elements, the electrical resistanceof resistor elements of a row relatively near the source of the coolingdraft being less than the electrical resistance of resistor elements ofa row relatively remote from the source of such draft so as to tend tbequalize the operating temperature of the resistor elements when acurrent of electricity is passed therethrough.

3. An electrical resistor comprising a plurality of resistor coilsarranged generally in rows and columns, the coils of each column beingelectrically connected together in multiple, the columns beingelectrically connected together in series, and means directing a coolingdraft through the coils generally transversely of the rows of coils, theelectrical resistance of coils of a row relatively near the source ofthe cooling draft being less than the electrical resistance of coils ofa row relatively remote from the source of such draft so as to tend toequalize the operating temperature of the coils when a current ofelectricity is passed therethrough.

4. An electrical resistor comprising a plurality of resistor coilsarranged generally in rows and columns, the axial dimension of the coilsbeing substantially uniform, the coils of the respective rows in eachcolumn being electrically connected together in multiple, the columnsbeing electrically connected together in series, and means directing acooling draft through the coils generally transversely of the rows ofcoils, the pitch of the respective coils and the transversecross-sectional area of the coiled elements forming the respective coilsbeing selected so that the electrical resistance of coils of a rowrelatively near the source of the cooling draft is less than theelectrical resistance of coils of a row relatively remote from thesource of such draft so as to tend to equalize the operating temperatureof the coils when a current of electricity is passed therethrough.

5. An electrical resistor comprising a plurality of resistor coilsarranged in spaced apart relationship in a twodimensional field, theaxial dimension of the coils being substantially uniform, meanselectrically connecting the coils to one another in multiple and meansdirecting a cooling draft through the coils, the pitch of the respectivecoils and the transverse cross-sectional area of the coiled elementsforming the respective coils being selected so that the electricalresistance of coils subjected to the cooling draft when such draft isrelatively cool is less than the electrical resistance of coilssubjected to the cooling draft when such draft is relatively hot andcoils subjected to the cooling draft when such draft is relatively coolhave relatively Wide openings between their convolutions in contrast tocoils subjected to the cooling draft when such draft is relatively hotto reduce obstruction to the cooling draft in its passage to coilssubjected to the cooling draft when such draft is relatively hot, so asto tend to equalize the operating temperature of the coils when acurrent of electriciy is passed therethrough.

6. An electrical resistor comprising a plurality of resistor elementsarranged in generally parallel columns, the elements of each columnbeing electrically connected together in multiple, the columns beingelectrically connected together in series, and means directing a coolingdraft through the elements generally parallel to the columns, theelectrical resistance of elements relatively near the source of thecooling draft being less than the electrical resistance of elementsrelatively remote from the source of such draft so as to tend toequalize the operating temperature of the elements when a current ofelectricity is passed therethrough.

References Cited by the Examiner UNITED STATES PATENTS 561,294 6/96Thomas 338-299 1,138,659 5/15 Huenerfauth `338-299 X 1,491,194 4/24Burger 338-218 X 1,563,363 l12/25 Hibbard 338-58 1,957,227 5/ 34 Reimerset al 338-299 X 2,234,289 3/41 Tenney 33:8*51 2,596,327 5/52 `Cox et al.219-381 2,858,402 10/58 Griffes et al 219-539 X 2,904,764 9/59 Minter338-218 X 3,102,970 9/ 63 Haskell et al. S23- 123 FOREIGN PATENTS633,284 12/49 Great Britain.

RICHARD M. WOOD, Primary Examiner. LLOYD MCCOLLUM, Examiner.

6. AN ELECTRICAL RESISTOR COMPRISING A PLURALITY OF RESISTOR ELEMENTSARRANGED IN GENERALLY PARALLEL COLUMNS, THE ELEMENTS OF EACH COLUMNBEING ELECTRICALLY CONNECTED TOGETHER IN MULTIPLE, THE COLUMNS BEINGELECTRICALLY CONNECTED TOGETHER IN SERIES, AND MEANS DIRECTING A COOLINGDRAFT THROUGH THE ELEMENTS GENERALLY PARALLEL TO THE COLUMNS, THEELECTRICAL RESISTANCE OF ELEMENTS RELATIVELY NEAR THE SOURCE OF THECOOLING DRAFT BEING LESS THAN THE ELECTRICAL RESISTANCE OF ELEMENTSRELATIVELY REMOTE FROM THE SOURCE OF SUCH DRAFT SO AS TO TEND TOEQUALIZE THE OPERATING TEMPERATURE OF THE ELEMENTS WHEN A CURRENT OFELECTRICITY IS PASSED THERETHROUGH.